Solid-state imager and method for manufacturing same

ABSTRACT

A solid-state imager is disclosed wherein isolation regions ( 4 ) are covered with power supply lines ( 8 ), a light-transmitting lens film ( 24 ) whose surface forms continuous convex portions above the isolation regions ( 4 ) convex towards channel regions ( 5 ) is provided, and a light-transmitting material having a refractive index lower than that of the lens film ( 24 ) is provided over the lens film ( 24 ).

TECHNICAL FIELD

The present invention relates to a solid state imager with improvedlight receiving efficiency, and a manufacturing method thereof.

BACKGROUND ART

FIG. 15 is a plan view showing an outline of a construction of aconventional frame transfer type solid state imager. The frame transfertype solid state imager 1 comprises; an imaging section 1 i, a storagesection 1 s, a horizontal transfer section 1 h, and an output section 1d. The imaging section 1 i comprises a plurality of vertical shiftregisters arranged in parallel with each other in the verticaldirection, and each bit of these vertical shift registers constitutes alight receiving pixel. The storage section is comprises a plurality ofvertical shift registers which continue to the plurality of verticalshift registers which constitute the imaging section 1 i. The horizontaltransfer section 1 h comprises a single row horizontal shift registerprovided on the output side of the storage section is, each bit of whichis associated with a line in the plurality of vertical shift registers.The output section 1 d comprises enough capacity to receive theinformation charges output from the horizontal transfer section 1 h.

In this construction, the information charges generated in the pluralityof light receiving pixels which constitute the imaging section 1 i arestored for a predetermined period in the light sensitive pixels, and arethen transferred at high speed to the storage section is in response toa frame transfer clock φf. The information charges are then storedtemporarily in the storage section 1 s, and transferred sequentiallyline by line to the horizontal transfer section 1 h in response to avertical transfer clock φv. The information charges transferred to thehorizontal transfer section 1 h are then transferred to the outputsection 1 d, sequentially pixel by pixel in response to a horizontaltransfer clock φh, and converted sequentially to a voltage value andoutput as a picture signal Y(t).

FIG. 16 is a plan view showing a partial construction of the imagingsection 1 i, and FIG. 17 is a cross-sectional view along the line X-X inFIG. 16.

A P-type diffusion layer 3 which acts as the device region, is formedupon a primary surface of an N-type silicon substrate 2. A plurality ofisolation regions 4 infused with high concentrations of P-typeimpurities are arranged in parallel a fixed distance apart in thesurface region of this P-type diffusion layer 3. Between these isolationregions 4 are formed N-type diffusion layers, and a plurality of channelregions 5 which act as transfer channels for the information charges areformed. A plurality of polycrystalline silicon transfer electrodes 7 arearranged in parallel with each other on the plurality of channel regions5 via a gate insulation film 6 made of thin silicon oxide, so as toextend in the direction transverse to the plurality of channel regions5. Three-phase frame transfer clocks φf1 to φf3, for example, areapplied to these transfer electrodes 7, and the state of the potentialof the channel regions 5 is controlled by these clock pulses.

An interlayer insulating film made of the same material as the gateinsulation film 6 is formed over the plurality of transfer electrodes 7,and a plurality of power supply lines 8 made of aluminum, for example,are arranged on the interlayer insulating film so as to cover theisolation regions 4. The plurality of power supply lines 8 connect tothe transfer electrodes 7 via contact holes 11 formed at predeterminedintervals in the interlayer insulating film, at those points where theisolation regions 4 and the transfer electrodes 7 intersect. Forexample, in the case of three-phase drive, a contact hole is providedfor every third transfer electrode 7, and each power supply line 8 isconnected to every third transfer electrode. An additional interlayerinsulating film 9 is formed so as to cover the plurality of power supplylines 8, and furthermore a protective film 10 made of silicon nitride isformed over this interlayer insulating film 9.

DISCLOSURE OF THE INVENTION

In the case of the solid state imager described above, a plurality ofpower supply lines 8 are formed on the light receiving region so as tocover the isolation regions 4. The aluminum material used in theplurality of power supply lines 8 typically has a characteristic ofreflecting light. Consequently, of the light which is uniformly incidentonto the light sensitive region, the light which is incident onto thepower supply lines 8 is reflected at the surface of the power supplylines 8. Accordingly, a problem occurs in that the light which isincident onto the power supply lines 8 is not guided to the channelregions 5, and not incorporated as information charges.

The present invention comprises: a semiconductor substrate; a pluralityof channel regions arranged in parallel with each other a fixed distanceapart on a principal surface of the semiconductor substrate; a pluralityof isolation regions provided in gaps between the plurality of channelregions; a plurality of transfer electrodes arranged above thesemiconductor substrate so as to extend in a direction transverse to theplurality of channel regions; a plurality of power supply lines arrangedover the plurality of transfer electrodes along the plurality ofisolation regions; a light transmitting insulating film laminated ontothe plurality of transfer electrodes so as to cover the plurality ofpower supply lines; and light transmitting upper and lower lens filmslaminated onto the insulating film, wherein a film thickness of theinsulating film is thicker at a center of the isolation regions andthinner at a center of the channel regions, and the upper lens film isshaped such that a surface thereof forms continuous convex portionsabove the isolation regions convex towards the channel regions, and theupper lens film has a refractive index higher than that of a substanceprovided in a layer above the lens film.

Moreover, the manufacturing method of the present invention comprises: afirst step for arranging a plurality of channel regions in parallel witheach other a fixed distance apart on a principal surface of asemiconductor substrate, and forming a plurality of isolation regions ingaps between the plurality of channel regions; a second step for forminga plurality of transfer electrodes above the semiconductor substrate soas to extend in a direction transverse to the plurality of channelregions, and forming a plurality of power supply lines above theplurality of transfer electrodes so as to cover the isolation regions; athird step for laminating a light transmitting insulating film having apredetermined film thickness onto the plurality of transfer electrodes;a fourth step for forming a mask pattern which covers the plurality ofpower supply lines and extends along the plurality of channel regions onthe insulating film; a fifth step for etching the insulating filmanisotropically along the mask pattern, and thinning a film thickness ofthe insulating film along the plurality of channel regions; a sixth stepfor laminating a light transmitting lower lens film onto the insulatingfilm; a seventh step for forming concave portions over the isolationregions by etch back processing of the lower lens film; and an eighthstep for laminating a light transmitting upper lens film onto the lowerlens film, wherein the upper lens film has a refractive index higherthan that of a substance provided in a layer above the upper lens film.

According to the present invention, the surface of the upper lens filmfulfills the same function as a prism, and can guide the light incidentover the power supply lines, to the channel regions. As a result, thelight irradiated onto the light receiving region can be taken into thepixel region with a high level of efficiency, and converted intoinformation charges with a high level of light sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view describing an embodiment of the presentinvention;

FIG. 2 is a diagram showing ray tracing for a case where theconstruction of the present invention is employed;

FIG. 3 is a cross-sectional view describing a first step in a solidstate imager manufacturing method of the present invention;

FIG. 4 is a cross-sectional view describing a second step in the solidstate imager manufacturing method of the present invention;

FIG. 5 is a cross-sectional view describing a third step in the solidstate imager manufacturing method of the present invention;

FIG. 6 is a cross-sectional view describing a fourth step in the solidstate imager manufacturing method of the present invention;

FIG. 7 is a cross-sectional view describing a fifth step in the solidstate imager manufacturing method of the present invention;

FIG. 8 is a cross-sectional view describing an optional step in thesolid state imager manufacturing method of the present invention;

FIG. 9 is a cross-sectional view describing a sixth step in the solidstate imager manufacturing method of the present invention;

FIG. 10 is a cross-sectional view describing an initial stage of aseventh step in the solid state imager manufacturing method of thepresent invention;

FIG. 11 is a cross-sectional view describing a middle stage of theseventh step in the solid state imager manufacturing method of thepresent invention;

FIG. 12 is a cross-sectional view describing a final stage of theseventh step in the solid state imager manufacturing method of thepresent invention;

FIG. 13 is a cross-sectional view describing an eighth step in the solidstate imager manufacturing method of the present invention;

FIG. 14 is a cross-sectional view describing another embodiment of thepresent invention;

FIG. 15 is a plan view showing an outline of a construction of aconventional frame transfer type solid state imager;

FIG. 16 is a plan view describing the construction of the imagingsection; and

FIG. 17 is a cross-sectional view describing the construction of theimaging section.

BEST MODE FOR CARRYING OUT THE INVENTION

The construction in FIG. 1 shows an embodiment of a solid state imagerof the present invention, showing the same part as FIG. 17. In thisdiagram, an N-type silicon substrate 2, a P-type diffusion layer 3,isolation regions 4, channel regions 5, a gate insulation film 6,transfer electrodes 7, and power supply lines 8 are the same as thoseshown in FIG. 17. The present invention is characterized in that a powersupply line 8 covers a plurality of transfer electrodes 7, and thesurface of an upper lens film 24 forms continuous convex portions abovethe isolation regions 4 convex towards the channel regions 5.

The upper lens film 24 is made of an optically transparent materialwhich has a refractive index higher than that of a substance above theupper lens film 24. Moreover, although not shown in FIG. 1, when forminga protective film over the upper lens film 24, the protective film isformed from an optically transparent material so as to cover the entiresurface of the upper lens film 24, and is given a flat surface.

For example, it is possible to form the upper lens film 24 from siliconoxide with a refractive index of approximately 1.4 to 1.5, and then notform a protective film leaving air with a refractive index of 1, or itis possible to form the upper lens film 24 from silicon nitride with arefractive index of approximately 2, and then form the protective filmfrom silicon oxide with a refractive index of approximately 1.4 to 1.5.

In the case of the present embodiment, the boundary surface between theupper lens film 24 and the protective film forms a gentle curved shapefrom near the center of the isolation region 4 to partway over thechannel region 5, and forms a flat shape from one end of this curvedshape towards the center of the channel region 5.

In this manner, by forming an upper lens film 24 which is lighttransmitting and whose surface forms continuous convex portions abovethe isolation regions 4 convex towards the channel regions 5, and havinga light transmitting material with a refractive index lower than that ofthe upper lens film 24 over the upper lens film 24, the upper lens film24 can function as a prism above the power supply lines 8, guiding thelight incident over the power supply lines 8, to the channel regions 5.The surface of the upper lens film 24 forms a curved shape near thecenters of the isolation regions 4, and in particular, the angle formedbetween the surface of the upper lens film 24 and the surface of theN-type silicon substrate 2 is set so as to increase as it nears thecenter portion of the power supply lines 8. As a result, the light whichis incident vertically onto the surface of the N-type silicon substrate2 is refracted to a greater degree by the upper lens film nearer thecenter portions of the power supply lines 8, and a greater amount oflight can be guided efficiently into the channel regions 5.

Furthermore, if a light transmitting lower lens film 23 with arefractive index higher than that of an insulating film 22 is laminatedonto the transparent insulating film 22 whose surface thickensprogressively from the channel region 5 side towards the center of theisolation regions 4, the lower lens film 23 also functions as a prismabove the power supply lines 8, enabling the light incident over thepower supply lines 8 to be guided to the channel regions 5 with evenbetter efficiency. For the lower lens film 23 also, the angle formedbetween the boundary surface of the lower lens film 23 and theinsulating film 22, and the surface of the N-type silicon substrate 2 isset so as to increase as it nears the center portion of the power supplylines 8. As a result, the light which is incident vertically onto thesurface of the N-type silicon substrate 2 is refracted to a greaterdegree by the lower lens film as the light nears the center portions ofthe power supply lines 8, and a greater amount of light can be guidedefficiently into the channel regions 5.

In the present embodiment, silicon oxide film or silicon nitride filmwere given as an example of the material for the upper lens film 24 andthe protective film, but the present invention is not limited to thesematerials. That is to say, the upper lens film 24 may be any materialprovided that the material has a refractive index higher than that of asubstance above the upper lens film 24, and is optically transparent.Furthermore, in a case where a protective film is formed over the upperlens film 24, the upper lens film 24 should be made of a material whichhas a refractive index higher than that of the protective film, andwhich is optically transparent.

Moreover, for the lower lens film 23 also, an optically transparentmaterial with a refractive index higher than that of the insulating film22 is suitable, but the refractive index of the lower lens film 23 doesnot necessarily need to be higher than that of the insulating film 22.In addition, the upper lens film 24 and the lower lens film 23 do notnecessarily need to be made of the same material.

Furthermore, by adjusting the angle of the curved shapes appropriatelyin accordance with the refractive indices of the upper and lower lensfilms and the other materials, it is possible to obtain a similar effectto that of the present embodiment. For example, when the light incidentover the power supply lines 8 can be guided to the channel regions 5 toa sufficient degree by the upper lens film 24 functioning as a prism, itis possible to form the upper lens film 24, the lower lens film 23, andthe insulating film 22 all from silicon oxide or silicon nitride.

FIG. 2 is a diagram showing ray tracing for a case where the presentembodiment is employed. In this manner the light incident over the powersupply lines 8 is focused efficiently towards the channel region 5 side.

FIG. 3 through FIG. 13 are cross-sectional views of individual stepsillustrating the solid state imager manufacturing method of the presentinvention. These diagrams show the same parts as shown in FIG. 1.

Step 1: FIG. 3

The surface region of an N-type silicon substrate 2 is diffused withP-type impurities such as boron to form a P-type diffusion layer 3 whichacts as the device region. The inside of this P-type diffusion layer 3is then further infused selectively with P-type impurities to formisolation regions 4, and the gaps between the isolation regions 4 areinfused with N-type impurities such as phosphorus to form an N-typediffusion layer which acts as the channel regions 5.

Step 2: FIG. 4

The surface of the N-type silicon substrate 2 in which the isolationregions 4 and the channel regions 5 are formed, is subjected to thermaloxidation, thus forming a gate insulation film 6 made of silicon oxide.A polycrystalline silicon film is then formed over this gate insulationfilm 6 using a CVD (chemical vapor deposition) process. Thispolycrystalline silicon film is then patterned according to apredetermined shape which is transverse to the channel regions 5,thereby forming the transfer electrodes 7.

Step 3: FIG. 5

Silicon oxide is laminated onto the transfer electrodes 7 using a CVDprocess, thus forming a first layer of insulating film. Contact holes 11are formed in this first layer of insulating film at positions above theisolation regions 4. Aluminum is then laminated onto the first layer ofinsulating film, and patterned according to a predetermined shape toform the power supply lines 8.

Step 4: FIG. 6

A BPSG film is then laminated using a CVD process onto the first layerof insulating film in which the power supply lines 8 are formed, thusforming an insulating film 22 in combination with the first layer ofinsulating film. Because this BPSG film is subjected to an etchingprocess in a subsequent step, in this fourth step the film is formed toa thickness which is thicker than the maximum thickness of the finishedproduct. The surface of the BPSG film is then subjected to heattreatment to smoothen the surface of the insulating film 22.

Step 5: FIG. 7

A resist layer 31 is laminated onto the insulating film 22, and thisresist layer 31 is patterned along the power supply lines 8, thusforming a mask pattern 32 which covers the power supply lines 8. Ananisotropic etching process (for example dry etching) is then performedon the insulating film 22 using the mask pattern 32 as the mask, therebyreducing the film thickness of the insulating film 22 along the channelregions 5.

Optional Step: FIG. 8

The remaining mask pattern 32 on the insulating film 22 is removed, andan isotropic etching process (for example wet etching) is performed onthe insulating film 22 which was subjected to the anisotropic etchingprocess. By this isotropic etching process it is possible to form theinsulating film 22 such that the film thickness becomes progressivelythicker above the isolation regions 4 from the channel region 5 sidetowards the isolation region 4 side. In this manner, by using a methodin which an isotropic etching process is performed after firstperforming an anisotropic etching process, it is even possible to easilyform shapes with curved surfaces as shown in FIG. 1, for example. Thatis to say, the film thickness of the lower lens film 23 which derivesfrom the interlayer insulating film 22 can be set freely within theprocessing time of the anisotropic etching process, and the angle of thecurved portions of the lower lens film can be set freely within theprocessing time of the isotropic etching process. By suitably regulatingthese two etching processes, it is possible to form the desired shapeaccurately at a predetermined position over the power supply lines 8,even if the type of device is one where the width of the isolationregions 4 is extremely narrow, like a frame transfer type solid stateimager. Here, this step is optional.

Step 6: FIG. 9

A plasma CVD process is used to laminate silicon nitride onto thesilicon substrate 1 on which the insulating film 22 is formed, thusforming the lower lens film 23 covering the entire surface of theinsulating film 22. At this time, the surface of the lower lens film 23reflects the concave and convex shape of the insulating film 22, forminggentle protrusions above the power supply lines 8.

Step 7: FIG. 10 through FIG. 12

As shown in FIG. 10, the surface of the lower lens film 23 is renderedflat by applying a resist 33, for example, to the lower lens film 23.Subsequently, the surface of the resist 33 is etched back by means of ananisotropic etch back process. At this time, by selecting the mixingratio of the etching gas appropriately, it is possible to obtainconditions under which the lower lens film 23 undergoes etching morereadily than the resist 33. Consequently, as shown in FIG. 11 the partswhere the lower lens film 23 is exposed to the etching gas are etched toa greater degree than the resist 33. As a result, when the entire resist33 is etched away, the surface of the lower lens film 23 forms gentleconcave sections above the power supply lines 8 as shown in FIG. 12.

Step 8: FIG. 13

A plasma CVD process is used to laminate silicon nitride onto thesilicon substrate 2 on which the lower lens film 23 is formed, thusforming the upper lens film 24 covering the entire surface of the lowerlens film 23. At this time, the surface of the upper lens film 24 formsgentle concave sections above the power supply lines 8. It is evenpossible to easily form shapes with curved surfaces as shown in FIG. 1,for example. That is to say, by setting the conditions of the laminationof the silicon nitride by the plasma CVD process appropriately, the filmthickness of the upper part of the lens portion based on the upper lensfilm 24 can be set freely, and the angle of the curved portion of theupper part of the lens portion can be set freely.

If required silicon oxide is laminated using a plasma CVD process ontothe silicon substrate 2 on which the upper lens film 24 is formed, toform a protective film covering the entire surface of the upper lensfilm 24. The surface of the protective film is then rendered flat bymeans of an etch back process or a CMP (Chemical Mechanical Polish)process.

According to the manufacturing method above, it is possible to obtainthe solid state imager shown in FIG. 1 which has the lower lens film 23and the upper lens film 24.

FIG. 1 shows the construction of an embodiment of the solid state imagerof the present invention manufactured by following the above steps 1through 5, the optional step, and steps 6 through 8. However, theconstruction showing an embodiment of the solid state imager of thepresent invention manufactured without undergoing the optional step isshown in FIG. 14. In this construction also, by providing above theupper lens film 24 a light transmitting substance with a lowerrefractive index than that of the upper lens film 24, the upper lensfilm 24 functions as a prism above the power supply lines 8, and thelight incident over the power supply lines 8 can be guided to thechannel regions 5.

In addition to frame transfer type devices, the present invention can beapplied of course to CCD type solid state imagers which use othertransfer methods, as well as to MOS type, BBD (bucket-brigade device)type, and CID (charge injection device) type solid state imagers, and tointensified solid state imagers such as avalanche type devices.

According to the present invention, by forming a light transmittingupper lens film whose surface forms continuous convex portions above theisolation regions convex towards the channel regions, and providing alight transmitting substance over the upper lens film which has arefractive index lower than that of the upper lens film, the upper lensfilm can function as a prism above the power supply lines, guiding thelight incident over the power supply lines, to the channel regions.Consequently, photoelectric conversion of the light irradiated onto thesemiconductor substrate can be performed efficiently, and lightsensitivity can be improved.

1. A solid state imager comprising: a semiconductor substrate; aplurality of channel regions arranged in parallel with each other afixed distance apart on a surface of said semiconductor substrate; aplurality of isolation regions provided in gaps between said pluralityof channel regions; a plurality of transfer electrodes arranged abovesaid semiconductor substrate so as to extend in a direction transverseto said plurality of channel regions; a plurality of power supply linesarranged over said plurality of transfer electrodes along said pluralityof isolation regions; a light transmitting insulating film laminatedonto said plurality of transfer electrodes so as to cover said pluralityof power supply lines; and a light transmitting lens film laminated ontosaid insulating film, wherein a film thickness of said insulating filmis thicker at a center of said isolation regions and thinner at a centerof said channel regions, and said lens film is shaped such that asurface thereof forms continuous convex portions above said isolationregions convex towards said channel regions, and said lens film has arefractive index higher than that of a substance provided in a layerabove said lens film.
 2. A solid state imager according to claim 1,wherein a film thickness of said insulating film becomes progressivelythinner above said isolation regions towards said channel regions.
 3. Asolid state imager according to of claim 1, wherein said lens film has arefractive index higher than said insulating film.
 4. A method ofmanufacturing a solid state imager, comprising: a first step forarranging a plurality of channel regions in parallel with each other afixed distance apart on a surface of a semiconductor substrate, andforming a plurality of isolation regions in gaps between said pluralityof channel regions; a second step for forming a plurality of transferelectrodes above said semiconductor substrate so as to extend in adirection transverse to said plurality of channel regions, and forming aplurality of power supply lines above said plurality of transferelectrodes so as to cover said isolation regions; a third step forlaminating a light transmitting insulating film having a predeterminedfilm thickness onto said plurality of transfer electrodes; a fourth stepfor forming a mask pattern which covers said plurality of power supplylines and extends along said plurality of channel regions on saidinsulating film; a fifth step for etching said insulating filmanisotropically along said mask pattern, and thinning a film thicknessof said insulating film along said plurality of channel regions; a sixthstep for laminating a light transmitting lower lens film onto saidinsulating film; a seventh step for forming concave portions over saidisolation regions by etch back processing of said lower lens film; andan eighth step for laminating a light transmitting upper lens film ontosaid lower lens film, wherein said upper lens film has a refractiveindex higher than that of a substance provided in a layer above saidupper lens film.